The invention relates to positive electrode material used for Li-ion batteries, a precursor and process used for preparing such materials, and Li-ion battery using such material in its positive electrode.
Compared to Ni—Cd and Ni-MH rechargeable batteries, Li-ion batteries boast an enhanced energy density, mainly due to their higher 3.6 V working voltage. Since their commercialization in 1991 by SONY, Li-ion batteries have seen their volumetric energy density increase continuously. In 1995, the capacity of a typical 18650 cylindrical cell was about 1.3 Ah. In 2006, the capacity of the same type of cell is about 2.6 Ah. Such a high energy density has enabled a wide range of applications. Li-ion batteries have become the dominant secondary battery for portable application, representing a market share of about 70% in 2006.
Such significant increase of energy density of Li-ion batteries has been initially realized by optimizing cell design, accommodating more active electrode materials in a fixed volume cell. Later efforts concentrated on improving the energy density of the electrodes. Using a high density active electrode material is one way to achieve this goal. As LiCoO2 still continues to be used as positive electrode material for the majority of commercial Li-ion batteries, a highly dense variety of this material is in demand.
The tap density of electrode materials is usually a good indicator of electrode density. However, in some cases, a high tap density does not guarantee a high electrode density. For example, as demonstrated by Ying et al. (Journal of power Sources, 2004) or in CN1206758C, the tap density of a LiCoO2 powder with large secondary spherical particle size, but small primary size, can be as high as 2.8 g/cm3. However, because of its small primary particle size, and possibly because of voids in the secondary particles, the obtained electrode density is not correspondingly high. For this reason, density of electrode materials should preferably be measured under a pressure similar to the industrial conditions prevailing during actual electrode manufacture, instead of by tapping. In this invention, density therefore refers to press density, and not to tap density.
The theoretical density of LiCoO2 is about 5.1 g/cm3. For actual LiCoO2 powders, factors that impact the density are a.o. the shape of particles, the size of primary particles and the particle size distribution. In today's industry, the median primary particle size of LiCoO2 used for different application is in the range of 1 to 20 μm. Generally, the larger the median primary particle size (d50), the higher is the press density. In addition, as proposed in CN1848491A, electrode density can be increased further by mixing larger LiCoO2 particles with 15 to 40 wt % of finer particles.
Besides density reasons, a large median primary particle size is also desirable for safety purposes, especially for large cells such as the 18650 model cylindrical cells that are used in laptop computer. During charge, lithium atoms in LiCoO2 are partially removed. LiCoO2 becomes Li1-xCoO2 with x>0. At high temperatures caused by certain abuse condition, Li1-xCoO2 tends to decompose and then to release O2. The released O2 easily reacts with organic solvent in the battery electrolyte, resulting in fire or explosion of the battery. Using LiCoO2 with a large median primary particle size and low specific surface area (BET) reduces these risks, as pointed out by Jiang J. et al. (Electrochimica Acta, 2004).
Therefore, for both safety and energy density reasons, LiCoO2 with large median primary particle size, such as 15 μm or above, is preferred, in particular for large Li-ion cells. Materials with a large mass median primary particle size (d50) have also a relatively low BET. A d50 larger than 15 μm typically leads to a BET below 0.2 m2/g.
In a usual manufacture process of LiCoO2, powderous Co3O4 and Li2CO3 are mixed and then fired at a temperature ranging from 800° C. to 1100° C. The d50 of the Co3O4 needs to be relatively small, usually below 5 μm, to ensure a sufficient reactivity. The growth of the LiCoO2 particles is controlled by the firing temperature and time, and by the amount of excess Li (added as Li2CO3). To make LiCoO2 with a d50 larger than 15 μm, at least 6 at. % of excess Li per Co atom is needed, as this excess favours crystal growth. However, part of the excess Li also enters the LiCoO2 structure. Therefore, the final product will be Li over-stoichiometric. This is why all current LiCoO2 material with large primary particle size (or a low BET, which is equivalent) is significantly over-stoichiometric. Due to this excess Li in their structure, such materials have a lower capacity because some active Co3+ has been replaced by inactive Li+. In this respect, it should be noted that in this application, LiCoO2 is used to designate a wide variety of lithium cobalt oxides having stoichiometries that may slightly deviate from the theoretical.
One example of this process can be found in EP 1 281 673 A 1. Here a composition Li Co(1-x)MgxO2 is disclosed, wherein x is 0.001 to 0.15, and having an average particle diameter of 1.0 to 20 μm and a BET of 0.1 to 1.6 m2/g. However, the examples clearly show that the inventor did not succeed in manufacturing a lithium cobalt (magnesium) oxide powder having both of: a d50 of more than 15 and a specific surface area (BET) of less than 0.2 m2/g. The maximum d50 achieved in this document is 8.3 μm in a comparative example.
It is finally also desirable for electrode materials to provide good rate capability. Rate capability is defined as the ratio of specific discharge capacity at a higher discharge rate (typically 2 C), to the specific discharge capacity at a lower rate (typically 0.1 C). Unfortunately, current LiCoO2 with large primary particle size shows relatively poor rate capability, as shown in JP3394364 and by Chen Yan-bin et al. (Guangdong Youse Jinshu Xuebao, 2005). Such poor rate capability is considered to be related to the longer Li diffusion path for material with larger primary particle size when Li is removed or reinserted during charge or discharge.
In summary, LiCoO2 with a large primary particle size is preferred for Li-ion battery for improved safety and energy density. However, current large particle size powders show sub-optimal capacity and rate capability because of the significant Li-excess in their structure.